I used ORD's shackle flip on the rear with stock springs and am completely happy with the results and ride quality. Chaski is spot-on regarding the pinion rotation calculation. On my M1028, the 4 degrees of rotation seemed to be perfect regarding the pinion angle, however because I also installed ORD's Zero-Rates, I had to lengthen my rear drive shaft a couple inches for proper spline engagement depth at the transfer case.
However I will also state that, on my truck, the pinion rotation also caused an unexpected issue pertaining to shock attachment which nobody had mentioned. I am not sure how the rear shocks on the M1009's are positioned, but on my M1028 the rear shock on the drivers side attaches to the frame in a forward direction and the shock on the passenger side attaches to the frame in a rearward direction. For whatever reason, when GM designed the position of the bottom shock mounting brackets, which are welded to the axle tube, they inexplicably placed them below the center-line of the tube. Because the shackle flip causes the axle tube to rotate, the passenger side shock mount on the axle tube also rotates, thereby causing the bolt hole for the bottom shock attachment to be too far under the tube to be used. When attempting to install the passenger side shock, I could bolt the bottom of the shock to the axle tube, but then could not attach the top bolt because the shock body would hit the axle tube. I considered flipping the shock over and installing the shock upside down, because in this configuration the wide shock body would be pointed up and the narrow shock piston would be pointed down. This would allow the shock to be attached, but just barely. I ruled this option out because I am not a fan of upside-down shock absorbers. After thinking about the situation a few days, I elected to cut the bottom shock mount off the axle tube and re-weld it back onto the tube in a higher position, thereby providing clearance beween the shock body and axle tube. After getting this done, and while admiring my work when looking from the rear of the truck, I decided to do the drivers side shock mount the same way because my conscience could not tolerate the out-of-balance look. After seeing how ground clearance was improved by moving the shock mounts, I then ordered and installed ORD's U-bolt flip kit. Now, with both bottom shock mounts re-welded onto the tube above the tube center-line and the U-bolt ends and nuts moved to the top of the spring packs, there is nothing hanging down to snag anything. In my opinion, GM should have designed the shock mounts and U-bolts this way originally.
While you are at it, I will also recommend replacing the spring bushings with ORD's grease-able poly units to eliminate the binding and squeaking of the old factory bushings.
Regarding the rusty springs, why don't you simply take them apart of restore the leafs? I did this and it was quite easy. I covered the entire process in posts #45 through #51 in my rebuild thread.
Hope this helps.
